How to reduce the number of faces for selected parts of a mesh?
Go into Sculpt Mode. 

Pick any brush (for example "draw") and set its strength to 0.

Enable "Dyntopo" from the Sculpt toolshelf and set Density = Brush in the Dyntopo options.

Now, control the size of your brush using F and "Sculpt" on the areas where you want to reduce the number of faces. One quarter of the brush size will be the approximate size of the new faces. Use undo CtrlZ when you need to!